1. Technical Field
The present invention pertains to the field of elevator control. More particularly, the present invention pertains to an active roller guide controller used to control the motion of an elevator transverse to the rail guides it rides on.
2. Description of Related Art
One type of active roller guide (ARG) system uses actuable springs (unparallel) and an electromagnetic actuator having an airgap between the poles of its electromagnet and a reaction bar slidably attached to a rail guide. The actuator is attached to the elevator. Current in the windings of the electromagnet produces magnetic flux that extends into the airgap. The square of the magnetic flux density in the airgap is directly related to the force of attraction between the electromagnet and the reaction bar, and hence between the elevator and the rail guide.
These active roller guides now sometimes use a pair of electromagnets to generate forces in opposite directions along a control axis at the location of the roller guides. The prior art active roller guides of this sort use flux feedback from each electromagnet. A force control loop, sometimes implemented using an analog computer, steers to the appropriate magnet a force dictation (a command to produce a specified force), depending on which direction the elevator is to be forced. In this prior art, each electromagnet always carries a minimum current, called here an idling current, even when not called upon to deliver force.
Because the windings of an actuator's electromagnet are finite in conductivity, all actuators are current-limited, and hence are also force limited. There is a maximum current the windings can carry, and hence a maximum force an actuator can produce. The force provided by an electromagnet is a nonlinear function of both the winding current and the airgap; it increases with the square of the current, and is inversely proportional to the square of the airgap.
When an elevator is forced away from a desired position on a control axis, the airgap for one actuator increases while that for the other decreases. When an airgap is at the large end of an operating range, typically at about 12 mm, the maximum force that can be generated is typically about 250N before a typically 10 A current limit is reached. At the opposite extreme, when the airgap is at the small end of the operating range, typically about 2.0 mm, assuming that the actuator magnet is idling at a typically minimum idling current of 1.0 A, the force produced by that idling current will be larger than 250N. When the system enters this configuration, the controller cannot free it. This locking up is called magnet stiction.
Essentially, stiction tends to develop in the prior art because a minimum idling current based control system is unstable with respect to holding an elevator at any location on a control axis away from both rail guides, so that neither airgap is too small. A minimum idling current amounts to a variable idling force, because the force depends on the airgap, which can vary; if the airgap decreases, then for the same current, the force produced by the magnet increases. This increasing force represents magnet stiction; it must be overcome by a larger current in an opposing magnet. But the opposing magnet has a larger airgap corresponding to the smaller airgap of the first magnet; and to produce an opposing force equal in magnitude to the force of the first magnet, a very much larger current is necessary. Thus, the minimum idling current based system is unstable because control is current-limited.
Magnet stiction cannot be overcome simply by reducing the idling current for two reasons. First, the lower the idling current, the greater the delay before a magnet can respond to a command to produce a certain level of force. Second, another component of an active roller guide, namely a centering controller, uses current feedback to calculate the lateral position of the elevator, and if too small an idling current were used, then at large airgaps, the flux feedback would be too small for reliable position calculation.
What is needed is a control system that avoids this unstable behavior caused by using a minimum idling current for each magnet.